1. Field of the Invention
This invention relates to a method and a sensor for determining the air/fuel ratio of a combustion mixture by measuring exhaust gas composition and more particularly to such a method and sensor which generate a signal proportional to the air/fuel ratio of such mixtures over a wide range of both rich and lean mixtures.
2. Prior Art
It has become desirable to determine the air/fuel ratio of combustion engine intake air/fuel mixtures to improve engine efficiency and reduce exhaust gas air pollution. Many combustion engine exhaust gas sensors in use today, such as the zirconium dioxide and titanium dioxide sensors which operate through thermal or deliberate catalytic action, or both, exhibit switch-like characteristics. That is: they generate one output for air/fuel mixtures rich of stoichiometry and another value lean of stoichiometry. Such sensors have drawbacks from a control system point of view since they do not allow the full mathematical processing abilities of the electronic control unit to be utilized. A two-state sensor that switches about the stoichiometric air/fuel ratio does not provide information for the employment of fuel enrichment techniques and lean zone operation. Moreover, a two-state sensor can be given only limited authority because such a control is inherently unstable and must oscillate. This application is limited to the three-way catalytic converter and duel bed emission control systems and is not extendable to the control of diesel or lean-burn control systems. Attempts to operate the zirconia and titania sensors over a range of oxygen partial pressures without catalysis, have encountered formidable difficulties, especially strong temperature sensitivity.
U.S. Pat. No. 3,933,028 discloses a sensor for detecting the air/fuel ratio of lean combustion engine intake mixtures. The partial pressure of oxygen present in the exhaust gas changes the electrical resistance of a cobalt monoxide ceramic sensor element. This change in resistance is used to generate an output signal which is a function of the partial pressure of oxygen in the lean regime. This device is only suitable, however, for generating signals representative of lean air/fuel ratios since the partial pressure of oxygen in combustion engine exhaust gases remains fairly constant for rich air/fuel intake mixtures.
U.S. Pat. No. 3,948,081 suggests that the steep slope of the signal in the area of stoichiometry produced by the prior art oxygen sensors can be smoothed out and a continuous signal representative of the air/fuel ratio can be generated by an electronic circuit which provides separate amplification factors for lean, rich and stoichiometric conditions. One difficulty with this approach is that, as mentioned above, the partial pressure of oxygen in combustion engine exhaust gases remains fairly constant for rich intake mixtures and hence is not a suitable parameter for determining the air/fuel ratio of such mixtures.
It is known that palladium has a high solubility for hydrogen and that the electrical resistance of a film of palladium will vary upon exposure to hydrogen by an amount corresponding to the hydrogen concentration. A hydrogen sensor operating on this principal is disclosed in the commonly owned U.S. Pat. No. 3,242,717. U.S. Pat. No. 3,138,948 discloses a hydrogen sensor in which a palladium containing resistance element generates an imbalance in a Wheatstone bridge circuit proportional to the partial pressure of hydrogen present in a test gas. A second resistance element covered with a hydrogen impervious coating provides temperature compensation for the bridge. Both of these sensors are used for the direct measurement of hydrogen partial pressures.
It is also known that hydrogen gas generators have been suggested for stabilizing electrochemical gas sensors. U.S. Pat. No. 4,025,412 discloses an electrochemical sensor in which the stabilizing hydrogen gas is generated within the sensor. A bias potential applied across the cell generates hydrogen ions at the sensing electrode from moisture in the test gas. The hydrogen ions are transported through the electrode to the reference electrode where they are reduced to molecular hydrogen and absorbed by an adjacent palladium foil. In this manner, an essentially constant partial pressure of internally generated hydrogen is maintained at the reference electrode. Catalytic action at the sensing electrode generates the sensor EMF while the hydrogen gas stabilizes the reference electrode potential at zero volts.
The primary object of the present invention is to provide an air/fuel mixture sensor which is continuously operative over a wide range of air/fuel mixtures from rich through lean, including stoichiometric. Yet another object is to provide such a sensor which is relatively insensitive to variations in temperature.